Printed circuit boards are commonly fabricated using the reflow solder technique. A paste containing solder particles mixed with flux, adhesives, binders, and other components is applied to selected areas of a printed circuit board. Electronic components are pressed against the applied solder paste. Adhesives in the paste hold the components to the printed circuit board. A conveyor belt within a reflow oven carries the printed circuit board and components through a high temperature region within the oven where they are heated to a temperature sufficient to cause the solder particles in the paste to melt. Molten solder wets metal contacts on the components and printed circuit board. Flux in the solder paste reacts with the contacts to remove oxides and to enhance wetting. The conveyor moves the heated printed circuit board to a cooling region of the oven where the molten solder solidifies forming a completed electronic circuit. An example of a typical reflow solder oven is the Vitronics M-series Reflow Soldering System, available from Vitronics Corporation, 2 Marin Way, Stratham, N.H., the assignee of the present investors.
The reaction of the flux with the contacts liberates vapors. Further, heat within the oven vaporizes unreacted flux as well as the adhesives, binders, and other components of the solder paste. The vapors from these materials accumulate within the oven leading to a number of problems. If the vapors migrate to the cooling region they will condense on the circuit boards, contaminating the boards and making subsequent cleaning steps necessary. The vapors will also condense on cooler surfaces within the oven, clogging gas orifices, gumming up moving parts, and creating a fire hazard. This condensation may also drip onto subsequent circuit boards destroying them, or making subsequent cleaning steps necessary. In addition, condensed vapors may contain corrosive and toxic chemicals which can damage equipment and create a hazard to personnel.
The vapors generated by the reflow operation collectively are referred to in this application as "flux vapors." It is understood that the flux vapors include vaporized flux, vapors from other components of the solder paste, reaction products released when the flux is heated, as well as vapors outgassed from the printed circuit board and electronic components.
Flux vapors can be flushed from the oven by providing a fresh supply of gas. This is not an ideal solution. In many cases the oven must be filled with an inert gas, for example nitrogen. Generating additional inert gas to flush the oven is expensive.
Several methods have been proposed for filtering flux vapors from the oven atmosphere. U.S. Pat. No. 5,579,981 (Matsumura et al.) cycles a portion of the oven atmosphere through an apparatus which cools the oven gases with a heat exchanger. The cooled vapors condense on the surface of the heat exchanger forming a liquid. The cooled gas is then reheated and any remaining vapors are combusted on a catalytic surface. The vapors that condense on the heat exchanger flow into a collection device for disposal.
U.S. Pat. No. 5,611,476 (Soderlund et al.) removes flux by cooling gas from the oven containing flux vapors on the surface of a heat exchanger, condensing the flux vapors. The cooled gas then passes through a filter to trap any remaining vapors not collected on the heat exchanger. The heat exchanger is either cleaned or replaced at intervals to remove the condensed flux vapors.
Each of these methods suffers from certain problems. Flux gases are a mixture of many components with a range of condensation temperatures, viscosities, and degrees of crystallization or polymerization. Further, the mixture of flux gas components will vary depending on what type of solder paste is used for a particular assembly. It is impractical to rely on the condensed vapors forming a free-flowing liquid, that will drip from the heat exchange as suggested by Matsumura et al. Under typical reflow soldering conditions, the heat exchanger of Matsumura et al. will become covered with solidified or highly viscous condensed flux vapors reducing its efficiency and eventually rendering it incapable of condensing additional vapors. These will have to be removed or the heat exchanger must be replaced.
Replacement of contaminated heat exchangers, as suggested by Soderlund et al., adds cost to the operation of the reflow oven. Either the oven must be shut down to remove the old heat exchanger and connect a new one, reducing the productivity of the oven, or parallel flux collection systems must be installed so that one system operates while the other is being serviced, adding cost to the oven. Since the heat exchanger must be connected with a working fluid, such as chilled water, replacement is complicated and requires the services of a skilled technician.
Cleaning of the heat exchanger is difficult given the nature of the condensed flux vapors. The condensed vapors are generally not water-soluble so that a solvent is required to remove them. Many solvents are toxic and/or flammable, presenting a safety hazard to workers. Disposal of solvent waste is expensive, particularly when the solvent waste includes a variety of unknown reaction products and other chemicals from the condensed flux vapors.